In ultrahigh-speed communications exceeding 100 Gbit/second, recent interest has been focused on a communication technology by DP-QPSK (Dual Polarization Differential Quadrature Phase Shift Keying) excelling in wavelength use efficiency, receiving characteristics, and dispersion compensation capability. A receiver for DP-QPSK system requires function to separate optical signal into polarized waves, and 90-degree optical hybrid function for retrieving phase information from the polarized waves. The phase information hereof is phase information of four values on I-Q plane: Ip and In having a phase difference of π, and Qp and Qn having a phase difference of π/2 to Ip and In respectively.
Receivers having such functions are becoming the standard in OIF (Optical Internetworking Forum), an industry group promoting high-speed data communications. For example, the specification of sequences of a port which outputs eight output signals is decided in OIF, and receivers are developed in accordance with the specifications of OIF.
Incidentally, it is said that a planar lightwave circuit using optical waveguide technology is influential for manufacturing a device which realizes the function of the receiver of such DP-QPSK system. Optical waveguide technology is for forming optical waveguides of various shapes on a substrate by the same micro fabrication technology as the semiconductor integrated circuit manufacturing process, and it is appropriate for integration or for mass production.
Receivers of DP-QPSK system by such lightwave circuit have been developed in recent years. For example, in the related art document (Toshikazu Hashimoto, et al., “Polarization Dual Optical Hybrid Modules Using Planar Lightwave Circuit”, The Institute of Electronics, Information and Communication Engineers, Electronic Society Conference, collection of conference papers 1 (2009), p. 194), a lightwave circuit structure shown in FIG. 7 is disclosed. On the lightwave circuit of FIG. 7, the above mentioned polarization separation function and the 90-degree optical hybrid function in a planar optical circuit are integrated, and the composition of the planar optical circuit which has the 90-degree optical hybrid function is shown in FIG. 8 as an exemplary diagram.
In the optical circuit hereof, four optical waveguide arms composes an interferometer for TE (Transverse Electric) optical signal and TM (Transverse Magnetic) optical signal. The 90-degree optical hybrid function is realized with an optical path length difference of a specific arm among the four arms longer than the other arms by ¼ of the signal optical wavelength. In the 90-degree optical hybrid interferometer disclosed in the above mentioned non-patent document, an arm 63 and an arm 65 are longer than the other arms by the above mentioned optical path length difference. Thus, when the optical path length difference is given as described, an interfering signal is outputted as shown in FIG. 9 to inputted TE optical signal and TM optical signal as four optical signals each having a phase difference of 702. In FIG. 9, the horizontal axis represents a phase difference between the TE optical signal and the local oscillator light, and InTE and QnTE are outputted together with a phase difference of π/2, and the same for IpTE and QpTE. Further, InTE and IpTE are outputted together with a phase difference of π, and the same for QnTE and QpTE. Similarly, InTM and QpTM are outputted together with a phase difference of π/2, and the same for IpTM and QpTM. In addition, InTM and IpTM are outputted together with a phase difference of π, and the same for QnTM and QpTM. Further, the arrangement of ports where such output signals are outputted as shown in FIG. 8 follows the specification decided in OIF.